Data have accrued implicating interleukin-18 (IL-18), a cytokine discovered for its immune system-modulating properties, in the control of food intake and energy metabolism. We and others found that the IL-18 modulates energy homeostasis during adulthood in mice. Mice deficient in IL-18 (IL-18-/-) or its binding receptor subunit (IL-18Ra) develop maturity-onset obesity and insulin resistance. As young adults, before they become substantially overweight, IL-18-/- mice are fattier, show reduced energy expenditure, and overeat both low-fat and high-fat diets as compared to wildtype mice. Because heterozygote IL-18+/- mutant mice show an intermediate hyperphagic phenotype, the findings appear to be regulatory, rather than an artifact of non- specific developmental lesion effects. Converse to these loss-of-function studies, central administration of IL- 18 suppresses food intake and weight regain in hungry mice as potently as does the adipocyte hormone leptin and does so without producing the adverse malaise, fever or HPA-axis activation classically associated with proinflammatory cytokine-driven sickness anorexia. Human genetic studies have recently identified single nucleotide polymorphisms (SNPs) in the gene for IL-18 or its receptor associated with increased risk for obesity and metabolic syndrome disorders. IL-18 and its receptors are constitutively expressed in the hypothalamus, with levels increased by peripheral immune challenge. Therefore, the guiding hypothesis tested in the present application is that central IL-18 systems can control food intake and energy expenditure. Translationally, this function would make IL-18 a clinically relevant target for sickness syndrome/cachexia. Moreover, because IL-18 administration does not share sickness syndrome effects of the pro-inflammatory system, exogenous IL-18 may have translational relevance as an anti-obesity treatment. To test this hypothesis, the present interdisciplinary proposal combines complementary but distinct, expertise of two PI's to determine mechanistically how central IL-18-responsive systems modulate food intake, energy expenditure, and, ultimately, adiposity. Experiments in two Specific Aims combine global and Cre/lox-targeted genetic loss- of-function mouse models deficient in IL-18 or the IL-18R; classic molecular, pharmacological, and physiologic approaches; and innovative behavioral analyses. AIM 1 seeks to identify physiologic and receptor mechanisms for central IL-18 control of food intake and energy expenditure. The potency and IL-18R- dependence of central IL-18 reductions in food intake and increases in energy expenditure will be determined. Pair-feeding is used to determine the functional role of excess food intake in the obesity of IL-18 null mutants. Feeding microstructure analysis is used to explore whether IL-18 modulates meal size or post-meal satiety in controlling food intake. Finally, the influence of diet-induced obesity on molecula expression and functional sensitivity of the central IL-18 system is determined. AIM 2 seeks to identify central sites and molecular mechanisms of IL-18 action on energy homeostasis. Expt. 2.1 will yield key information regarding the cell types the express IL-18 and its cognate receptors within the hypothalamus under basal conditions and following peripheral immune challenge (lipopolysaccharide), histochemically differentiating neuronal from glial populations, modes of fast synaptic transmission, and neuroanatomical interactions with anorectic pathways implicated in cytokine anorexia. In Expts 2.2-2.3, Cre/lox targeting will be used to generate mice that lack IL- 18R subunits selectively in the paraventricular nucleus of the hypothalamus (PVN) and the ventromedial hypothalamic nucleus (VMH). Studies will involve phenotyping the offspring for energy balance parameters to allow comparison with results obtained in the global knockout and also to determine whether the site-specific deletion alters feeding or metabolic responses to central IL-18 administration. The studies provide a powerful test of the functional significance of IL-18/IL-18R systems in the PVN and VMH for the control of food intake and energy expenditure. Collectively, the results will provide seminal data regarding the sites and mechanisms of action by which IL-18 systems modulate food intake and energy expenditure. Such information will be key for beginning to understand the pathophysiologic and potential therapeutic relevance of central IL-18 signaling for disorders of energy homeostasis, such as obesity or cachexia.